Since SiC has a high breakdown electric field, it can reduce the device thickness required for achieving a high breakdown voltage to about one-tenth compared with Si. Therefore, the Schottky diode having the breakdown voltage of 300 V or higher, which was unsuitable for practical use in the case of Si due to the large voltage drop which occurs when applying a current, has been realized.
A pn diode is generally used as a Si high breakdown voltage diode. The pn diode is a bipolar device using both electrons and holes as electric conduction carriers, and in the transition from a conduction state to a voltage blocking state corresponding to an off state, excess minority carriers (holes) accumulated in a low impurity concentration layer (generally, n layer) are discharged to generate a recovery current. This becomes a switching loss of a diode. In an inverter, switching devices and diodes are used, and a recovery current of the diode causes the turn-on loss of the switching device. On the other hand, since a Schottky diode is a unipolar device using only electrons (or holes) as carriers, a recovery current like in a pn diode is not generated, and it has the feature of being able to significantly reduce the switching loss of a diode and a switching device. This is the major purpose of applying the Schottky diode up to the high breakdown voltage level.
Meanwhile, a Schottky diode is a device in which rectification occurs by the Schottky junction of metal and semiconductor, and it is affected by built-in potential of the metal-semiconductor junction. For example, in the case of a SiC Schottky diode using Ti as an electrode, a forward voltage drop (hereinafter, referred to as FVD) of 0.9 V or higher is required for causing a current to flow in a forward direction, and it is virtually impossible to operate the SiC Schottky diode with the FVD of 1.0 V or lower. On the other hand, a Si pn diode requires FVD of about 0.6 V and can operate with 1.0 V or lower though current density is not high, and the Si-pn diode has lower loss when compared with respect to the conduction loss.
The structure for avoiding the influence of the built-in potential of the metal-semiconductor junction described above includes a pinch rectifier of Japanese Patent Application Laid-Open Publication No. 60-74582 (Patent Document 1) shown in FIG. 6, which was created for the purpose of improving the switching speed. In the Patent Document 1, the pinch rectifier is made up of a cathode electrode 521, an n+ cathode layer 510, an n− drift layer 511, n+ contact regions 516, p+ regions 512 disposed to be deeper than the n+ contact region 516 so as to surround the n+ contact region 516 and an anode electrode 522 being in ohmic contact with the n+ contact regions 516 and the p+ regions 512. Since the n+ contact region 516 and the anode electrode 522 are in ohmic contact, the influence of the built-in potential like in the conventional Schottky diode can be eliminated, and the operation with a lower FVD can be achieved. In a reverse-bias state in which the voltage of the cathode electrode 521 side is higher than that of the anode electrode 522, the breakdown voltage is ensured by the pinch-off caused by the depletion layer expanded from the p+ region 512 by the field effect, and this is the structure which can be called as a field-effect diode.